1. Field of the Invention
The present invention relates to an image forming apparatus including a toner stain removal device for removing a toner stain adhering to a rear surface of a sheet.
2. Description of the Related Art
In general, in image forming apparatuses for forming an image on a sheet by an electrophotographic process, such as a copying machine, a printer, and a facsimile, a toner image is formed on the sheet in a manner as described below.
First, a photosensitive member is charged to a predetermined polarity by a charger. Then, an electrostatic latent image which corresponds to an image formed on an original is formed on a surface of the photosensitive member. Then, the electrostatic latent image is developed by a developer which is arranged at a predetermined position around the photosensitive member. To be specific, the electrostatic latent image is developed with toner, which is a charged fine powder and is supplied from the developer, thereby obtaining a toner image. The toner image formed on the photosensitive member reaches a transferring part through rotation of the photosensitive member to be directly transferred onto a transfer material (hereinafter, referred to as “sheet”) which has been transported to the transferring part. In some types of image forming apparatuses, there is a case where a toner image is not directly transferred onto a sheet from the photosensitive member, but is temporarily transferred onto an intermediate transfer member before being transferred onto the sheet. In any type of image forming apparatuses, the toner image is transferred onto the transported sheet. In the transferring part, the sheet is transported to a fixing part while the toner image is transferred onto the sheet. In the fixing part, the sheet is heated and pressurized to thereby fix the toner image on the sheet. Finally, a sheet delivery part delivers the sheet onto a sheet stacking tray.
In such image forming apparatuses, there is a fear of a toner stain adhering to a surface on an opposite side (i.e., rear surface) of a sheet on which a toner image is formed.
In other words, in the transferring part for transferring a toner image onto a sheet by using a transfer roller, the transfer roller is always in contact with the photosensitive member or the intermediate transfer member (hereinafter, both of which are collectively referred to as “image bearing member”), so base-staining toner of the image bearing member is transferred onto the transfer roller, and then the toner adheres to the rear surface of the sheet, thereby staining the rear surface of the sheet. Therefore, in order to prevent the sheet from being stained, it is a serious problem how to prevent the toner stain from adhering to the rear surface of the sheet, or how to remove the toner stain when the toner stain adheres to the rear surface of the sheet. The stain is usually removed by bringing a cleaning member, which is arranged around the transfer roller, into contact with a surface of the transfer roller.
However, in a marginless image forming apparatus for forming a toner image on a sheet with no margin on the periphery of the sheet, there is a case where the apparatus does not exert a sufficient cleaning ability only by providing the cleaning member, thereby adhering an unnecessary toner stain to the rear surface of the sheet.
The marginless image forming apparatus has a structure in which a toner image which is larger in size than a sheet is formed on the image bearing member. As a result, a portion of the toner image which runs off the edge of the sheet is transferred onto the transfer roller. A toner amount of this case is large as compared with a normal amount of the base-staining toner, so the cleaning ability of the cleaning member is not sufficient to clean the toner, which results in causing adhesion of a toner image to the rear surface of the sheet as the toner stain.
Therefore, there is conventionally provided a toner stain removal device for removing the toner stain adhering to the rear surface of the sheet (see FIG. 4 of JP 2001-42664 A). The toner stain removal device has such a structure that a sheet is brought into contact with a toner removal roller which serves as a cleaning member and is arranged between a transferring part and a fixing part and on the transferring part side, by a suction force of a fan, thereby cleaning directly the rear surface of the sheet through rotation of the toner removal roller. It should be noted that the toner stain adhering to the rear surface of the sheet is not fixed onto the sheet because the sheet has not passed the fixing part, thereby being removed through rotation of the toner removal roller.
However, in the toner stain removal device described in JP 2001-42664 A, a contact state of the sheets with respect to the toner removal roller vary between a thick sheet and a thin sheet because the suction force of the fan is kept constant. Accordingly, it is difficult to remove the toner stain adhering to some sheets with a certain thickness.
In addition, when the suction force of the fan is kept constant, in a case where an image forming mode is set to a mode of forming images on both surfaces of the sheet, the following problem arises.
Because the sheet is heated in the fixing part upon passing the fixing part, there is a fear of a toner image being curled (i.e., warps) to an opposite side of a surface onto which the toner image is transferred. As a result, when the toner image is formed on a second surface of the sheet subsequently to a first surface thereof, the sheet is turned over by a sheet reverse path, so a direction of the curl when the sheet passes the toner removal roller is a direction in which the sheet is apart from the toner removal roller.
Therefore, in the conventional toner stain removal device, a contact property of the sheet with respect to the toner removal roller deteriorates when the sheet the second surface of which has been transferred with the toner image, passes the toner removal roller of the toner stain removal device, thereby making it difficult to reliably remove the toner stain adhering to the first surface of the sheet.
Further, in the image forming apparatus including the toner stain removal device by which the toner stain adhering to the rear surface of the sheet is not reliably removed, there is a problem in that it is difficult to provide a sheet having a toner image formed thereon with less staining.